1. Field of the Invention
The present invention relates to a light emitting device with a three-dimensional structure (hereinafter, referred to simply as a “three-dimensional light emitting device”) using semiconductor nanoparticles and a method for fabricating the light emitting device. More specifically, the present invention relates to a three-dimensional light emitting device with improved luminescence efficiency wherein a substrate is provided with three-dimensional recesses and the entire surface of the recesses is coated with semiconductor nanoparticles, and a method for fabricating the light emitting device.
2. Description of the Related Art
With the recent advances in digital communication technologies, there has been increasing demand for highly functional and efficient photonic products. Since the 1990's, great efforts have been directed toward the development of light emitting devices using semiconductors.
In light emitting devices using semiconductors, when an electric current is applied to a p-type semiconductor and an n-type semiconductor joined to each other, electrons of the n-type semiconductor present in a conduction band of the energy bands recombine with holes of the p-type semiconductor present in a valence band of the energy bands to release energy corresponding to the energy gap between the valence and conduction bands in the form of light.
Generally, quantum confinement effects are utilized in light-emitting layers of light emitting devices to enhance the luminescence efficiency of the light emitting devices. That is, electrons and holes of the conduction band are confined in an active layer of a quantum well structure, and as a result, the state density of the carriers in the quantum well is increased, thus leading to an effective increase in the luminescence recombination efficiency of the electrons and the holes. In addition, since the refractive index of the quantum well is larger than that of a semiconductor material surrounding the quantum well, photons generated in the quantum well are also spatially confined in the vicinity of the quantum well. Light emitting devices can be used in a wide variety of photonic products, including displays (e.g., flat panel displays), screens (e.g., computer screens) and medical devices requiring irradiation. Accordingly, high luminance, low operation voltage, and high efficiency of light emitting devices are important factors in determining the quality of the photonic products.
In recent years, a number of studies on quantum dot displays have been undertaken in view of high luminescence efficiency. Quantum dot displays are devices wherein semiconductor rods having a size on the order of several nanometers are formed and light emission is achieved by tunneling effects. The advantage of quantum dot displays is that light emitting diodes (“LEDs”) having a size of several nanometers are densely distributed to emit light, thus achieving markedly improved luminescence efficiency.
FIG. 1 is a schematic cross-sectional view of a conventional quantum dot light emitting device as an organic electroluminescence device. As shown in FIG. 1, the conventional quantum dot light emitting device comprises a substrate 10, a pair of electrodes 20 and 60 formed over the substrate 10, a semiconductor nanoparticle layer 40 interposed between the electrodes 20 and 60, and a hole transport layer 30 and an electron transport layer 50 formed on the upper and lower surfaces of the semiconductor nanoparticle layer 40, respectively. Most light emitting devices that are currently in use have a structure in which constituent elements thereof, such as electrodes and light-emitting layers, are evenly formed. In such light emitting devices with a planar structure, however, a large proportion of light generated from the light-emitting layer is totally reflected from the surface of a substrate or electrodes and is confined in the devices, thus causing a decrease in the amount of light emitted from the devices.
To solve the problem, attempts have been made to develop structures capable of releasing light generated from a light-emitting layer to the outside without any loss of the light. For example, U.S. Patent Application Publication No. 2003/0057417 discloses an organic light emitting device in which a photonic crystal concavo-convex structure is formed in a transparent substrate to generate a leaky wave, thereby increasing the light extraction efficiency. According to the light emitting device, however, loss of the internally generated light is reduced but there is a limit to the increase in the internal light. In addition, another limitation of the light emitting device is that it is difficult to form the photonic crystal structure in a large area, in an economical manner.
Further, Korean Laid-open Patent No. 2005-0025919 (“the '919 Patent”) discloses a high-luminance organic light emitting device having a structure in which a plurality of hemispherical concave portions, particularly nanometer-sized hemispherical concave portions, are continually formed in a substrate and/or electrodes. According to the organic light emitting device, since the shape of the concave portions is limited to hemispheres, methods for forming the concave portions are limited. In addition, a drawback associated with the formation of the concave portions by oxidation of Al, which are suggested in the '919 patent, is that it is difficult to control the shape of the concave portions.
Further, Japanese Unexamined Patent Publication No. 2004-87615 discloses a method for manufacturing a semiconductor laser comprising forming a semiconductor layer on a substrate, forming regular grooves on the semiconductor layer, and aligning quantum dots on the bottoms of the grooves in a three-dimensional direction to form a light-emitting layer. According to this method, however, since the quantum dots are formed on the bottoms of the grooves by a vapor deposition process, such as molecular beam epitaxy, the use of expensive equipment is required, incurring considerable manufacturing costs.